The Champion

January-February 2012 , Page 58 

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Retrograde Extrapolation: A Scientifically Flawed Procedure (DWI)

By Dominick A. Labianca, Ph.D.

Read more DWI columns.

Retrograde Extrapolation: A Scientifically Flawed Procedure

If a driver’s blood-alcohol concentration (BAC) is below the legal limit when the driver gives a breath or blood sample, this fact will not stop the prosecution from trying to obtain a DWI conviction. At trial, the state may attempt to use retrograde extrapolation to convince the jury that the BAC exceeded the legal limit earlier when the police stopped the driver on the highway. But retrograde extrapolation is an unreliable tool.

According to the concept of retrograde extrapolation, a blood-alcohol concentration derived from the analysis of a subject’s breath or blood sample at a particular test time1 can be extrapolated back to the supposedly higher BAC existing at an earlier incident time. This is accomplished by adding to the BAC at test time the product of the hourly rate of alcohol elimination from blood (commonly termed the β value, as per Widmark2) and the number of hours elapsed between incident and tes

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